The present invention relates to an improved ceramic foam filter for filtering molten metal, in particularly, iron and iron based metal alloys, and a process for preparing the same. More particularly, the present invention relates to ceramic foam filters having silicon carbide as the ceramic component, which is used in the filtration of iron and iron based alloys.
It is known in the art to employ porous ceramic foam filters to filter molten metal, especially aluminum, as described in U.S. Pat. Nos. 3,893,917 and 3,947,363. The materials for these aluminum filters often comprise primarily a phosphate bonded refractory material having certain other additions which have been fired to a temperature of about 2000xc2x0 F. in order to mature the bond. While this type of refractory material is suitable for use in the aluminum industry, which easily withstands most aluminum alloys, which are typically cast to 1300xc2x0 F., it is unsuitable for many other potential applications, such as iron and iron based alloys, due to low strength, poor chemical resistance and poor high temperature stability. It is thus desirable to use a material which maintains the favorable properties of ceramic foam aluminum filters, namely, high porosity, low pressure drop, high geometric surface area and tortuous path, but overcomes the foregoing difficulties of strength, chemical durability and temperature resistance.
U.S. Pat. No. 4,610,832 describes an improved ceramic foam filter and a process for preparing the same, which is particularly useful for high temperature applications, such as ferrous or steel filtration, based on the use of an aqueous slurry of thixotropic ceramic composition including a gelled alumina hydrate binder. It has been found that for certain applications, specialized running and gating systems are needed to insure priming of this filter, although, the filter does represent a significant improvement.
U.S. Pat. No. 4,975,191 describes a ceramic foam filter which is prepared from a ceramic slurry containing silicon carbide and colloidal silica (hereinafter referred to as xe2x80x9cEarly SiC Filterxe2x80x9d). The Early SiC Filter has been very successful in filtering iron and ferrous alloys at high temperatures, normally up to 2700xc2x0 F. The Early SiC Filter is readily primeable as a result of the wetting affect of the colloidal silica and has improved thermal properties.
The Early SiC Filter also contains a large amount of alumina, normally approximately 24% of the dry ingredients and 16% of the entire slurry, which acts as a binder for the silicon carbide. While alumina binder works well for filters used to filter molten aluminum, it is believed that the use of alumina in a silicon carbide iron filter lowers the strength, in particularly the hot strength, of the filter.
More recently, silicon carbide filters which utilize a smaller alumina content have been produced (hereinafter xe2x80x9cLater SiC Filterxe2x80x9d). Later SiC Filters contain a smaller amount of alumina, normally 6% of all of the dry ingredients and 3% of the entire slurry batch. The Later SiC Filters utilize a large amount of fused silica and a small amount of fumed silica as a binder which reduces the amount of alumina needed. One of the problems with both the Early SiC Filter and the Later SiC Filter is that the minimum density of the filters are limited due to insufficient strength. In order to provide a part with acceptable strength characteristics, one should create a part that has no less than 10% of the theoretical density. For certain applications, this density is not adequate. As a result, there is a minimum thickness that can be manufactured due to the strength limitations.
In addition, both Early SiC Filters and Later SiC Filters suffer from so-called rheology problems. That is, the slurry tends to run while no work is being performed on the slurry. When the organic foam used in the manufacture of the filter is impregnated with the slurry, it has been found that some of the slurry falls towards the bottom of the foam due to gravitational effects. This affects performance because one side of the filter can clog up during manufacture. It is believed that the use of alumina has adverse effects on rheology because of the need to use more water than is necessary to produce adequate flow. Thus it is desirable to provide an improved silicon carbide filter which overcomes the problems of the prior art.
As used herein, fumed silica means a material primarily made of silicon dioxide having an average particle size of less than 15 microns, and fused silica means a material primarily made of silicon dioxide having an average particle size of greater than 15 microns.
It is therefore one object of this invention to provide an improved silicon carbide ceramic foam filter.
It is another object of this invention to provide a silicon carbide ceramic foam filter having improved mechanical properties.
It is still another object to provide a silicon carbide ceramic foam filter which is produced from a slurry having improved rheology.
It is yet another object to provide a silicon carbide ceramic foam filter which may have a low theoretical density while maintaining strength.
It is also another object to provide a silicon carbide ceramic foam filter which has high breakage resistance at both hot and room temperatures.
In accordance with one form of this invention there is provided a ceramic foam filter for use in filtering molten metal. A filter body is provided, which is prepared from a ceramic slurry composition containing an amount of silicon carbide, an amount of colloidal silica, and at least 10% fumed silica. The filter body has an open cell structure with a plurality of interconnected voids surrounded by a web of ceramic. The amount of fumed silica in the slurry is such that the strength of the filter is substantially enhanced.
In accordance with another form of this invention there is provided a process for preparing a ceramic foam filter for filtering molten metal. An aqueous slurry of a ceramic composition is formed including an amount of silicon carbide, an amount of colloidal silica and at least 10% of fumed silica. A reticulated organic foam is provided. The foam is impregnated with the slurry. The impregnated foam is dried and heated to remove the organic component. The dried foam is fired at an elevated temperature to produce the ceramic foam filter.